To Hippocamp: Neptune’s smallest moon has a name (and a history of violence)

An artistic illustration of Netpune the smallest moon Hippocamp in an orbit around the gas giant. It is only 20 miles across.
(J. Olmsted (STScI)/NASA/ESA)

A soft and cold small moon doesn’t have to go by “Neptune XIV” more.

Astronomers have a name — “Hippocamp” — the most recently discovered moon of Neptune, that also formerly went through S/2004 N1. They have figured out how big the satellite is, and teased some interesting details about his past, a new study reports.

A team led by Mark Showalter, of the SETI (Search for Extraterrestrial Intelligence) Institute in Mountain View, California, announced the existence of S/2004 N1 in 2013. The scientists did that after analyzing photos taken by NASA’s Hubble Space Telescope between 2004 and 2009. [See Photos of Neptune, The Mysterious Blue Planet]

When the team determined that S/2004 N1 is about 65,400 miles (105,250 kilometers) from the parent planet and completes one orbit every 23 hours or so. For comparison, Earth’s moon at 2,160 miles wide (3,475 km) a giant in comparison with the Neptune satellite orbits our planet at an average distance of about 239,000 miles (384,600 km).

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And the researchers linked S/2004 N1 as the smallest of Neptune’s 14 known moons, estimate the diameter not larger than 12 miles (19 km).

But things have changed a bit since then, as the new research — led by Showalter reports. The team has an update on the assessment of the moon, after the recording of the new Hubble observations made in 2016.

But let’s talk about the new name. “Hippocamp” is a horse head, fish-tailed creature in Greek mythology. The name, which is approved by the International Astronomical Union, in accordance with the naming convention for the Neptune system, that the question of a link with the Greco-Roman mythology and the sea. (Neptune, of course, is the Roman god of the sea, the equivalent of the Greek Poseidon.)

But the Hippocampus is also the name for the real life of seahorses. And Showalter is a diver who loves these beautiful and bizarre creatures.

“So, officially, it is named after this mythological creature,” said Showalter “But for a part, in my head, it is named after the seahorses, because I think they’re cool.”

In the new Hippocamp analysis, which was published today (Feb. 20) in the journal Nature, Showalter and his team using a smart technique that they invented a few years back (which they were able to discover that the moon in the first place). The scientists “transformed” eight consecutive 5 minutes with the Hubble images of the Neptune system, the rearranging of pixels, so that they can “stack” images of Hippocamp on top of each other, despite the moon’s orbital motion.

Essentially, the researchers proved to the eight individual positions in a 40-minute exposure.

“We came very close to the absence of the whole,” Showalter said of Hippocamp. “It is too faint to see in a single Hubble – [exposure].” [The Moons of Neptune Unmasked! (Infographic)]

This technique is powerful; the application of it in the general “could result in the detection of other small moons around giant planets, or even planets in orbit around distant stars ,” astronomer Anne Verbiscer of the University of Virginia, who was not part of Showalter’s team, wrote in an accompanying “News and views” piece in the same issue of Nature.

The new analysis is a slightly larger world than previously thought: Hippocamp is now believed to have a diameter of approximately 21 miles (34 km), the researchers report. That is about the same size as Ultima Thule, the strange and distant object that NASA’s New Horizons spacecraft flew by on New Year’s Day.

Hippocamp circles in the same area as six moons discovered by NASA’s Voyager 2 spacecraft during the probe’s flyby of Neptune in 1989. Hippocamp is only 7,450 miles (12,000 km) interior of the largest and most remote of the other six, the 260-kilometer-wide (420 km) Proteus.

Like Earth’s own moon, Proteus, is slowly rising away from its parent planet, for centuries — and so has Hippocamp, albeit at a much slower pace. About 4 billion years ago, Proteus was probably right next to Hippocamp and therefore would have swallowed up the smaller moon, Showalter said.

So, he and his colleagues suspect that the Hippocamp is younger than Proteus. In fact, they believe that the smaller moon was once a part of its larger neighbor: Hippocamp probably coalesced from pieces of Proteus that were blasted into space by a long-ago comet impact, the researchers wrote in the new Nature paper.

Indeed, Hippocamp able to trace the origin to the smashup that created Proteus’ massive Pharos Crater. Hippocamp total volume is about 2 percent of that is ejected during the Pharos impact. It is not difficult to imagine that this small amount of material beautiful shape of a moon, Showalter said.

In the 1980’s and ’90’s, astronomers began to posit that the moons of the giant planets endured a number of comet collisions, so a lot of the satellites to break apart. The origin derived from Hippocamp supports this view of the early solar system, said Showalter, who has played an important role in the discovery of the many natural satellites in the course of the years, including Saturn “ravioli moon” Pan in the early 1990s.

“This is the first really big example of a moon that was created as a result of a collision,” he said.

Showalter and his team also made use of the transformation stack technique to spot the Neptune moon Naiad, who has not been seen since its discovery by Voyager 2 in 1989. And the researchers are a number of restrictions on the possibility of finding other moons of the ice giant: Their analyses suggest that there are no moons larger than 15 miles (24 km) interior to Proteus, and not the least 12.4 miles (20 km) wide than that of the same satellite, Verbiscer noted.

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